1. Field of the Invention
The present invention relates to an indicator such as a tachometer which can be suitably used to measure the number of revolutions of an engine or a speed of a vehicle detected as a rotational speed, a thermometer, an ammeter, a voltmeter, and the like.
2. Description of the Prior Art
Conventional indicators of this type are, for example, an indicator using a cross coil indicator as disclosed in Japanese Utility Model Laid-Open No. 59-144567, an indicator using a movable coil indicator as disclosed in Japanese Utility Model Laid-Open No. 59-134071, and an indicator using a stepping motor.
Of the above conventional indicators, according to the cross coil indicator, a frequency must be converted into a voltage and two-phase sine wave and cosine wave currents must be supplied to a coil with respect to a voltage signal. However, sine wave/cosine wave conversion is very difficult and costly, and hysteresis is increased very much due to abrasion and the like of a bearing portion when the amplitude of each wave is around 0. In addition, linearity is degraded by imbalance of a rotor system including a pointer, also the accuracy of the electrical circuit and magnetic circuit are adversely affected by temperature changes, resulting in very poor indicating accuracy.
The movable coil indicator has problems as described in Japanese Utility Model Laid-Open No. 59-144567. In addition, a bearing portion is abraded, linearity is degraded by imbalance of a rotor system including a pointer, and accuracy of all the circuits including the magnetic circuit is degraded by temperature changes and deterioration over time, resulting in very poor indicating accuracy. Moreover, since fine adjustment is required, the indicator becomes expensive.
In addition, the movable coil indicator indicates at a point where a torque obtained by an input value and applied on a pointer is balanced with a spring incorporated in the indicator. Therefore, in order to improve accuracy, the operational speed of the pointer is reduced. Moreover, nonlinear regions are always present at upper and lower limits of an indicating range. Therefore, in order to obtain accurate indication, the indicator must not be used in these regions but in a narrow range with good linearity. In addition, since no force acts on the pointer at the point where a torque balance between the input value and the spring is obtained, indicating variations tend to occur due to vibrations and the like. The above-mentioned indicating linearity often depends on the type of spring used in the indicator. For this reason, a hair spring with relatively high linearity is used, resulting in high cost and variations in products caused by temperature changes. Therefore, indicating accuracy of only about 1% can be obtained.
Since the indicator using a stepping motor is generally controlled by a microcomputer or the like, not only a motor but also a control circuit becomes expensive. In addition, when an indicator of this type is applied to, e.g., a vehicle, the motor tends to be detuned due to intense vibration or impact and hence an indication error occurs. In this case, the system cannot be recovered until it is reset.